Metal porous bodies have been used thus far in various applications, for example, filters required to have heat resistance, electrode plates for batteries, and furthermore, catalyst supports, and metallic composite materials. Therefore, manufacturing techniques for metal porous bodies have been known through many publicly known literatures. Furthermore, products using CELMET (registered trade name) manufactured by Sumitomo Electric Ind., Ltd., which is a Ni-based metal porous body have been widely used in the industry.
The conventional metal porous body is produced by forming a metal layer on the surface of a foamed resin, etc., and thereafter, firing and removing the resin portion while the metal layer is reduced. For example, according to the method described in Japanese Unexamined Patent Application Publication No. 57-174484, after the surface of a porous core material, for example, a foamed resin, is subjected to a treatment for imparting electrical conductivity, a metal layer is formed by a plating method. For example, according to the method described in Japanese Examined Patent Application Publication No. 38-17554, a slurry containing a metal powder is adhered to the surface of a core material made of a foamed resin, etc., and drying is performed so as to form a metal preliminary layer.
In the former method where the metal layer is formed by the plating method, the treatment for imparting electrical conductivity is performed by adhesion-application of a conductive material, evaporation of a material for imparting electrical conductivity, or surface modification with a chemical agent, etc. Subsequently, the metal layer finally to become a porous metal body is formed by electroplating, or electroless plating, etc. Ultimately, the metal porous body is produced by firing and removing the resin portion which is a porous core material. In the case where an alloyed porous body is produced, different sorts of metal plating layers are formed, and they are subjected to a metal diffusion treatment by heating.
In the latter method, a slurry containing a metal powder and a resin is prepared in advance, which becomes the metal preliminary layer. In this method, an alloy powder or a mixed metal powder composed of a plurality of metals having an alloy composition is used as the metal powder of the slurry, and the porous metal body, which is alloyed by heating after drying, can be produced.
However, regarding the alloyed porous metal body produced as described above, since the adhesion property among the particles of the metal powder is particularly degraded due to oxidation or deterioration of the surfaces of the particles, the mechanical strength of the porous body decreases as compared with that of the porous metal body produced by the former method in which the diffusion alloying treatment is combined after plating.
An example of improvement in terms of such drawback, targeting at a porous iron alloy body is disclosed in Japanese Patent Publication No. 6-89376. According to the method, a specified amount of carbon is contained in the iron powder prepared in advance in the slurry and, in addition, the surface thereof is forced to oxidize. This causes an oxidation-reduction reaction between the oxygen in the oxide and the carbon contained to occur during firing and, as a result, the adhesion property among the metal powder particles is improved.
In addition, a sintered iron porous body having a dense metal skeleton, the raw material of which is an iron oxide powder, is disclosed in Japanese Unexamined Patent Application Publication No. 9-231983. However, even this method, further modification of the metal itself is required in order to enable the porous body to be used as a structural member for which high mechanical strength, heat resistance, and wear resistance are important characteristics. For example, as described in the aforementioned publication, since the characteristics in terms of mechanical strength, corrosion resistance, and heat resistance are inadequate, the improvement of these characteristics is attempted by alloying.
Furthermore, use of porous metal bodies has been accelerated by combination with a casting such as an Al die casting. This combination technique is a method in which a casting of a light metal is melt-infiltrated into gap portions of the porous metal body, and has been used widely as a means for achieving weight reduction by changing the Al alloy to the casting. In this case, further improvement of the characteristic can be expected by alloying the portion primarily containing Al which is to be combined with a porous body primarily containing Fe. Consequently, the same is expected with respect to combination with an alloy of another light metal, for example, Mg.
The technique regarding combination using a metal porous body is disclosed in detail in Japanese Unexamined Patent Application Publication No. 9-122887. According to the description in this publication, such a combined light-metal alloy is used in particular for the part of harsh use, for example, a slide portion, etc. Consequently, the characteristics of the metal porous body itself used for combination are required to coincide with the uses.
The aforementioned CELMET has been used as the metal porous body used for combination with the light metal as described above. However, a technique for producing a material having further excellent performance is described in Japanese Unexamined Patent Application Publication No 10-251710. For producing the porous metal body, a coating of a slurry containing a metal powder and a ceramic powder is applied to a member made of a foamed resin capable of being burned off, and subsequently the resin component is burned off in a reducing gas atmosphere containing steam/or carbon dioxide, and furthermore, the temperature is raised so as to perform firing in the reducing atmosphere. As a result, ceramic particles are dispersed in the skeleton of the resulting porous metal body and, therefore, a porous metal body having superior ceramic characteristics is formed.
In addition to this, there is a porous metal body disclosed in Japanese Unexamined Patent Application Publication No. 8-319504, in which gaps among the powders are used, as a metal powder is molded and sintered to the extent that it does not become dense. In this method, the volume percentage of the porous metal body is 30% to 88%, which is higher than that of the present invention and, therefore, when combination with Al, for example, is performed, a high pressure is required in order to impregnate the interior of the porous metal body with an Al melt. Furthermore, since the proportion of the metal porous body in the composite material becomes large, there is a problem in that a merit of weight reduction is not exerted. Herein, the volume percentage refers to the volume percentage of the skeleton portion relative to the total volume of the porous body.
Some problems in the use of the metallic composite material have been overcome by research regarding metal combination techniques as described above. Recently, such a metallic composite material has been noted and has been used as a material for weight reduction of engine components of automobile, etc. However, regarding this sort of components, requirements of the materials have become increasingly stringent in terms of emission control, etc. For example, further excellent wear resistance has been required of components used for, especially, wear-resistant piston ring portions of diesel engines. A composite material using the aforementioned metal porous body containing ceramic particles is mentioned as a potential material for such a component. However, regarding this material, since ceramic particles are contained in the skeleton of the porous body, preform working becomes difficult compared with that of the common porous body composed solely of metal, and therefore, the shapes, which can be made by working, are restricted.
Most of all, in the case of a component, such as a bore material of an engine block, used under the high-speed sliding condition at a high temperature, improvements of wear resistance, excellent moldability capable of near net preform molding and, in addition, especially, seizing resistance against the sliding counterpart material are very important challenges.